A. Field of the Invention
This invention relates to the field of nondestructive inspection and particularly to the field of transducers for use in ultrasonic inspection.
B. Description of the Prior Art
Elastic or ultrasonic waves are commonly used to inspect parts for cracks and other defects. Generally, a narrow beam of a longitudinal or a transverse type wave is injected into the part by a transducer which directly contacts the part or indirectly contacts the part through a transmitting medium such as water which contacts both the transducer and the part being tested. A piezoelectric crystal in the transducer is used to generate the elastic wave.
More recently, non-contact type transducers have been developed which can generate elastic Lamb-type waves in an object of conductive material without any physical contact with the object. Non-contact transducers are particularly useful in applications where the transducer moves relative to the test object because it eliminates friction and wear of the transducer. One such non-contact transducer is the electromagnetic transducer described in U.S. Pat. No. 3,850,028. Such prior art non-contact transducer utilizes a meander coil placed within a static magnetic field to generate Lamb waves by means of a Lorentz force mechanism.
The prior art meander coil electromagnetic transducer can generate only a Lamb type wave which fills the entire cross-section of the part. While Lamb waves can be advantageously used for inspecting parts such as large diameter pipelines (see for example, patent application No. 731,199), they have low radiation impedance and consequently require strong magnetic fields and high currents to obtain strong signals. For applications involving a small confined space, such as the inspection of tubes from the inside, it is difficult to provide the strong magnetic fields and high currents required to obtain strong signals using Lamb waves generated by meander coil transducers.